Return Values

pthread_cond_destroy() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occur, the function fails and returns the corresponding value.

EINVAL

The value specified by cv is invalid.

The Lost Wake-Up Problem

Calling pthread_cond_signal() or pthread_cond_broadcast() when the thread does not hold the mutex lock associated with the condition can lead to lost wake-up bugs.

A lost wake-up occurs when:

• A thread calls pthread_cond_signal() or pthread_cond_broadcast().

• And another thread is between the test of the condition and the call to pthread_cond_wait().

• And no threads are waiting.

  The signal has no effect, and therefore is lost.

The Producer/Consumer Problem

This problem is one of the small collection of standard, well-known problems in concurrent programming: a finite-size buffer and two classes of threads, producers and consumers, put items into the buffer (producers) and take items out of the buffer (consumers).

A producer must wait until the buffer has space before it can put something in, and a consumer must wait until something is in the buffer before it can take something out.

A condition variable represents a queue of threads waiting for some condition to be signaled.

Example 4-11 has two such queues, one (less) for producers waiting for a slot in the buffer, and the other (more) for consumers waiting for a buffer slot containing information. The example also has a mutex, as the data structure describing the buffer must be accessed by only one thread at a time.

Example 4-11 The Producer/Consumer Problem and Condition Variables

typedef struct {
    char buf[BSIZE];
    int occupied;
    int nextin;
    int nextout;
    pthread_mutex_t mutex;
    pthread_cond_t more;
    pthread_cond_t less;
} buffer_t;

buffer_t buffer;

As Example 4-12 shows, the producer thread acquires the mutex protecting the buffer data structure and then makes certain that space is available for the item being produced. If not, it calls pthread_cond_wait(), which causes it to join the queue of threads waiting for the condition less, representing there is room in the buffer, to be signaled.

At the same time, as part of the call to pthread_cond_wait(), the thread releases its lock on the mutex. The waiting producer threads depend on consumer threads to signal when the condition is true (as shown in Example 4-12). When the condition is signaled, the first thread waiting on less is awakened. However, before the thread can return from pthread_cond_wait(), it must acquire the lock on the mutex again.

This ensures that it again has mutually exclusive access to the buffer data structure. The thread then must check that there really is room available in the buffer; if so, it puts its item into the next available slot.

At the same time, consumer threads might be waiting for items to appear in the buffer. These threads are waiting on the condition variable more. A producer thread, having just deposited something in the buffer, calls pthread_cond_signal() to wake up the next waiting consumer. (If there are no waiting consumers, this call has no effect.)

Finally, the producer thread unlocks the mutex, allowing other threads to operate on the buffer data structure.

Example 4-12 The Producer/Consumer Problem--the Producer

void producer(buffer_t *b, char item)
{
    pthread_mutex_lock(&b->mutex);
   
    while (b->occupied >= BSIZE)
        pthread_cond_wait(&b->less, &b->mutex);

    assert(b->occupied < BSIZE);

    b->buf[b->nextin++] = item;

    b->nextin %= BSIZE;
    b->occupied++;

    /* now: either b->occupied < BSIZE and b->nextin is the index
       of the next empty slot in the buffer, or
       b->occupied == BSIZE and b->nextin is the index of the
       next (occupied) slot that will be emptied by a consumer
       (such as b->nextin == b->nextout) */

    pthread_cond_signal(&b->more);

    pthread_mutex_unlock(&b->mutex);
}

Note the use of the assert() statement; unless the code is compiled with NDEBUG defined, assert() does nothing when its argument evaluates to true (that is, nonzero), but causes the program to abort if the argument evaluates to false (zero). Such assertions are especially useful in multithreaded programs--they immediately point out runtime problems if they fail, and they have the additional effect of being useful comments.

The comment that begins /* now: either b->occupied ... could better be expressed as an assertion, but it is too complicated as a Boolean-valued expression and so is given in English.

Both the assertion and the comments are examples of invariants. These are logical statements that should not be falsified by the execution of the program, except during brief moments when a thread is modifying some of the program variables mentioned in the invariant. (An assertion, of course, should be true whenever any thread executes it.)

Using invariants is an extremely useful technique. Even if they are not stated in the program text, think in terms of invariants when you analyze a program.

The invariant in the producer code that is expressed as a comment is always true whenever a thread is in the part of the code where the comment appears. If you move this comment to just after the mutex_unlock(), this does not necessarily remain true. If you move this comment to just after the assert(), this is still true.

The point is that this invariant expresses a property that is true at all times, except when either a producer or a consumer is changing the state of the buffer. While a thread is operating on the buffer (under the protection of a mutex), it might temporarily falsify the invariant. However, once the thread is finished, the invariant should be true again.

Example 4-13 shows the code for the consumer. Its flow is symmetric with that of the producer.

Example 4-13 The Producer/Consumer Problem--the Consumer

char consumer(buffer_t *b)
{
    char item;
    pthread_mutex_lock(&b->mutex);
    while(b->occupied <= 0)
        pthread_cond_wait(&b->more, &b->mutex);

    assert(b->occupied > 0);

    item = b->buf[b->nextout++];
    b->nextout %= BSIZE;
    b->occupied--;

    /* now: either b->occupied > 0 and b->nextout is the index
       of the next occupied slot in the buffer, or
       b->occupied == 0 and b->nextout is the index of the next
       (empty) slot that will be filled by a producer (such as
       b->nextout == b->nextin) */

    pthread_cond_signal(&b->less);
    pthread_mutex_unlock(&b->mutex);

    return(item);
}